This invention relates to hydraulic press equipment and, more specifically, it relates to a novel apparatus and method for supplying pre-fill hydraulic fluid to a hydraulic motor during its closure stroke and recovering the same during its retraction stroke.
Hydraulic press equipment generally includes a hydraulic motor consisting of a cylinder and a piston driven back and forth hydraulically. Such equipment is used for a variety of purposes such as holding the parts of an injection mold together during the injection of molding materials and for compressing materials into desired shapes and sizes. For a typical application such as press molding, the piston is operable from a "home" or fully retracted position through a closure stroke to a work position in which the mold parts are brought together, and beyond that, through a compression stroke to a fully extended or "closed" position and back again through a retraction stroke to its fully retracted position. Of course, the work for which the greatest squeezing pressure and energy is needed is done during the compression stroke. It takes relatively little energy to move the piston and its associated ram and mold parts through the closure and retraction strokes. It has therefore become a practice to use low energy and relatively low pressure hydraulic fluid for execution of the closure and retraction strokes and a high energy, relatively high-pressure hydraulic fluid for the compression stroke. In this way, very little high pressure hydraulic fluid is consumed and energy costs are therefore conserved.
Various apparatuses and method have been devised over the years to supply hydraulic fluid under low pressure to the power input side of the piston during execution of the closure stroke. Fluid supplied this way is generally called "pre-fill fluid". The system described in U.S. Pat. No. 3,915,614 to Farrell, for example, employs a reservoir from which hydraulic fluid is fed by gravity to the cylinder of a hydraulic motor as the closure stroke is being executed. When the piston is in position for execution of the compression stroke, the line to the reservoir is closed and high pressure hydraulic fluid is applied to the piston from a separate source driving it through the compression stroke. Upon execution of the retraction stroke hydraulic fluid is returned to the reservoir.
While arrangements like that described above function well enough, they are subject to certain disadvantages. First, rather large-sized and costly piping and valving is required between the reservoir and the hydraulic motor to ensure reasonably prompt transfer and return of the hydraulic fluid to and from the reservoir. Secondly, the energy required to drive the hydraulic fluid back into the reservoir is considerable. Thirdly, even with large piping, such systems tend to be fairly slow resulting in the substantial consumption of operator time. And finally, such systems do not conveniently deal with the excess volume hydraulic fluid that is transferred to the reservoir during the retraction stroke. Those skilled in the art realize that this excess volume hydraulic fluid occurs because the fluid is compressed somewhat during the compression stroke and then expands as it is transferred under relatively low pressure to the reservoir. Also, if the piston is further advanced during the compression stroke, more fluid is added to the cylinder which will be transferred to the reservoir. Although it is possible to pipe fluid from the reservoir back to the hydraulic power unit from which the high pressure hydraulic fluid is supplied, this arrangement is subject to the same shortcomings mentioned above.
It is therefore an object of this invention to provide an apparatus and a method for supplying prefill hydraulic fluid from a reservoir to a hydraulic motor and for returning that hydraulic fluid to the reservoir while expeditously expelling excess volume hydraulic fluid to the tank of a hydraulic power unit.
It is a further object of this invention to provide an apparatus and method of the character described in which relatively small and inexpensive piping and valving of conventional types may be used.
It is a further object of this invention to provide an apparatus and method of the type described in which the transfer of pre-fill hydraulic fluid to and from a reservoir is made in very little time, thus allowing a hydraulic press to execute cycles rapidly.
Finally, it is an object of this invention to provide a method and an apparatus of the type described in which very little energy is consumed in the transfer of pre-fill hydraulic fluid to and from the reservoir.
Briefly described, the apparatus of this invention employs a reservoir for holding a supply of hydraulic fluid for use in pre-filling a hydraulic motor during its closure stroke and recovering that hydraulic fluid when it is displaced by the motor during its retraction stroke. The reservoir is provided with a pre-fill fluid port connected to the motor and a hydraulic fluid drain outlet for discharging excess volume hydraulic fluid. Means are provided for controllably pressurizing the internal space of the reservoir. Control means connected in the hydraulic fluid drain outlet and the pre-fill fluid port and to the motor and the pressurizing means causes the hydraulic fluid to be transmitted to the motor under pressure as the piston executes its closure stroke and to be returned to the reservoir as it is displaced by the motor during its retraction stroke with any excess volume hydraulic fluid being expelled under pressure through the hydraulic fluid drain outlet.
Also briefly described, the method of this invention as applied to a hydraulic press comprises the steps of advancing the piston through its closure stroke, supplying a predetermined volume of pre-fill hydraulic fluid under relatively low pressure to the motor as the closure stroke is executed, driving the piston with relatively high pressure hydraulic fluid through its compression stroke, retracting the piston through its retraction stroke, collecting hydraulic fluid displaced from the motor as the piston executes its retraction stroke, reserving from the collected hydraulic fluid substantially the same volume as that which was supplied to the motor during the closure stroke and expelling under pressure any excess volume hydraulic fluid created during execution of the method.